Update documentation to fix Rubocop errors in 'rails generate' command

Previously, the 'rails generate' command would send unsupported files to
Rubocop,causing errors and failures in the process.
To fix this, we added a filter that only sends parsable
files to Rubocop.

This commit updates the documentation to reflect this change,
helping users avoid errors and complete
'rails generate' command successfully.

Follow-up to rubocop#961 and rubocop#956

README.md

@@ -71,7 +71,8 @@ your config/application.rb to apply RuboCop autocorrection to code generated by
 module YourCoolApp
   class Application < Rails::Application
     config.generators.after_generate do |files|
-      system("bundle exec rubocop -A --fail-level=E #{files.shelljoin}", exception: true)
+      parsable_files = files.filter { |file| file.end_with?('.rb') }
+      system("bundle exec rubocop -A --fail-level=E #{parsable_files.shelljoin}", exception: true)
     end
   end
 end

docs/modules/ROOT/pages/usage.adoc

@@ -41,7 +41,8 @@ your config/application.rb to apply RuboCop autocorretion to code generated by `
 module YourCoolApp
   class Application < Rails::Application
     config.generators.after_generate do |files|
-      system("bundle exec rubocop -A --fail-level=E #{files.shelljoin}", exception: true)
+      parsable_files = files.filter { |file| file.end_with?('.rb') }
+      system("bundle exec rubocop -A --fail-level=E #{parsable_files.shelljoin}", exception: true)
     end
   end
 end